1. Technical Field
The present invention relates to the field of controlling a wireless communication system like a WLAN or HIPERLAN/2 system and in particular to a method and a device for controlling frequency selection within a wireless communication system in response to radar-like interference signals.
2. Related Art and Other Considerations
The expression HIPERLAN stands for High Performance Radio Local Area Network. HIPERLAN/2 (H/2) is a standard for high speed radio communication with typical data rates up to 54 Mbit/s.
A H/2 network consists typically of a number of access points (AP). Each AP covers a certain geographic area. Together the APs form a radio access network with full or partial coverage of an area of almost any size. Each AP serves a number of mobile terminals (MT) which are associated to it. In the case where the quality of the radio link degrades to an unacceptable level, the MT may move to another AP by performing a handover.
Another operational mode is used to operate H/2 as an adhoc network without relying on a cellular network infrastructure. In this case a central control (CC), which is dynamically selected among the MTs, provides the same support as a fixed AP.
H/2 systems are intended to be operated in the 5 GHz frequency range. The nominal carrier frequencies of H/2 are allocated in two frequency bands. In the following, the frequency band between 5150 MHz and 5350 MHz will be called the lower frequency band and the frequency band between 5470 MHz and 5720 MHz will be called the upper frequency band. The nominal carrier frequencies within each frequency band are spaced 20 MHz apart.
H/2 systems and other wireless communications systems require dynamic frequency adaption—also called Dynamic Frequency Selection (DFS)—to local interference conditions. The task of DFS is to detect interference from other system in order to avoid co-channel operation with these systems. A possible realization of DFS is to periodically measure the interference on all used and not used frequencies and to control the frequency selection in accordance with the measurement. Thus, when for example a currently used frequency is suddenly disturbed by interference, a new frequency may automatically be selected which is less interfered than the frequency currently in use.
Possible solutions for DFS in wireless communications systems are periodically accomplished measurements during the normal mode of operation. In order to keep the transmission capacity high, measurements should only rarely be accomplished, e.g. a measurement takes place every few seconds. However, under certain circumstances such a measurement strategy does not work satisfactorily because of the following reasons.
H/2 systems for example must be able to share the upper frequency band and parts of the lower frequency band with radar systems, some of which are mobile. Typical radar systems use rotating antennas with a small main lobe of approximately 1° for horizontal scanning. Consequently, radar interferences are difficult to detect with DFS measurement strategies. This situation is depicted in FIG. 1, where WR and WH denote the interval length of a radar signal and a H/2 DFS measurement, TR and TH denote the period length of the radar signal detection and the H/2 DFS measurement, TP denotes the period length of the radar signal and LP denotes a radar pulse width.
Taking into account typical interval lengths and typical period lengths of a radar signal and a DFS measurement, the detection of a radar signal is not reliable enough.
There exists, therefore, a need for a method of controlling frequency selection within a wireless communication system in response to radar-like interference signals which allows to implement a highly effective detection strategy. There is also a need for a wireless communication system which performs such a detection strategy.